The present invention is directed to a method for separating and regenerating a mixed bed of anion and cation resins and to a composition formed during the practice of the method.
Mixed bed systems containing anion and cation exchange resins have many industrial applications. A primary application of such a system is in the purification of water for condensate recirculation systems used to drive steam turbines in fossil fuel and nuclear facilities. It is essential that this water be of an extremely high purity level in order to avoid any adverse effects on the surfaces of turbine blades, boilers, pipes, and other parts of the facility.
A particular problem with mixed bed ion exchange systems is the production of ion leakage. Ion leakage can result from the failure of the ion exchange resins in the mixed bed to remove ions from the water and the passage of the unremoved ions past the mixed bed. In addition, the introduction into the water of undesired ions by the resins themselves can also contribute to ion leakage.
The leakage of regenerant chemical ions from the resins arises primarily from the difficulty in perfectly separating the anion and cation resins in the mixed bed prior to regeneration of the resins. A conventional technique for effecting such separation is by passing water upwardly through the resins. This upward flow of water stratifies the resins by carrying the less dense anion exchange resin to the top of the separation vessel, while the denser cation exchange resin sinks to the bottom of the separation vessel. While this method is effective for separating the bulk of the resins into two stratas or layers, perfect separation cannot be achieved. A primary source for imperfect separation is resin fines that are produced during handling of the resins. Since upflow separation depends on the particle size as well as the density of the resins, the cation exchange resin fines do not sink to the bottom of the separation vessel, but are carried upwardly with the anion exchange resin. When the resins are subsequently isolated from one another, and the anion exchange resin is regenerated with sodium hydroxide, sodium ions can be introduced into the ion exchange sites in the entrained cation resin. When the resins are returned to the service vessel, the sodium ions are displaced from the cation resin by any ions present in the influent water that have a higher selectivity or affinity for the cation resin, e.g., ammonium. The displaced sodium ions are thereby introduced into the water being treated, producing sodium leakage.
Also, anion resins can be entrapped among the cation resin during the passage of water upwardly through the mixed bed of resins. When this occurs, the regeneration of the cation resin with sulfuric acid exhausts the entrained anion resin to the sulfate and bisulfate forms. When the resins are returned to the service vessel, the sulfate ions are introduced into the water being treated, thereby producing sulfate leakage. It is believed that sulfate leakage from the anion resin is primarily due to the bisulfate ions being driven off the anion resin by ammonium hydroxide which is typically used to control the pH of the influent water.
In addition to not achieving perfect separation, the conventional technique for separating anion and cation resins possesses several further disadvantages. For example, the size of the cation resin capable of being employed in the mixed bed is unduly restricted. This is because resin size, as noted above, as well as resin density, are factors which contribute to the efficiency of the conventional backwash separation procedure. Accordingly, the conventional backwash procedure requires that the cation resin be larger than the anion resin. This restriction limits the efficiency of the cation resin since larger resin size particles have less surface area per unit volume and therefore exhibit slower ion transfer rates or kinetics.
Furthermore, the vessel or zone through which the water is passed upwardly to stratify the resins typically has a fixed anion resin takeoff location or point. Accordingly, the bulk volume of cation resin employed in the mixed bed is restricted since the bulk volume of the cation resin must be sufficient to occupy the volume of the zone below the anion resin takeoff point, thereby causing the upwardly lifted anion resin to occupy a volume above the anion takeoff point.
Another commercially used method for separating and isolating exhausted anion and cation exchange resins is disclosed in U.S. Pat. No. 3,582,504. The method of U.S. Pat. No. 3.528.504 comprises first separating the resins in the conventional manner by passing a liquid upwardly through the resins to position the anion resin in an upper layer and the cation resin in a lower layer. The layers are then conventionally isolated from one another so that the anion exchange resin occupies an anion exchange resin zone and the cation exchange resin occupies a cation resin zone. It is estimated that the anion resin in the anion exchange resin zone generally contains less than about 10 percent volume/volume (% v/v) entrained cation resin and more typically less than about 5% v/v entrained cation resin.
The process of U.S. Pat. No. 3,582,504 is characterized in that an intermediate-density liquid is then delivered to the anion exchange resin to remove the entrained cation resin from the anion resin zone. This intermediate-density liquid has a density intermediate between the densities of the anion exchange resin and the cation exchange resin, i.e., greater than the anion exchange resin and less than the cation exchange resin. The intermediate density liquid is delivered to the anion exchange resin in an amount sufficient to cause the anion resin to float and the cation resin to sink. The separated anion exchange resin is then isolated from the entrained cation exchange resin.
U.S. Pat. No. 3,582,504 discloses that numerous intermediate-density liquids, e.g., organic liquids and aqueous solutions of inorganic compounds that have a density intermediate between the anion and cation exchange resins, can be employed. U.S. Pat. No. 3,582,504 specifically mentions sodium sulfate and alkali metal hydroxide solutions, the most preferred aqueous salt solution being sodium hydroxide.
There are several distinct disadvantages with the separation process of U.S. Pat. No. 3,582,504. First, the process employed in U.S. Pat. No. 3,582,504 still separates the bulk of the anion and cation resins by the conventional technique of passing water upwardly through the resins. As noted above, this separation process can leave entrapped anions in the cation resin and thereby can continue to contribute to sulfate leakage due to exhausting the entrained anion resins with sulfuric acid. Second, the cation resin entrained in the separated anion resin can be exhausted, e.g., to sodium, during the separation of the entrained cation resin from the anion resin, e.g , with sodium hydroxide. Even though the exhausted cation resin can later be regenerated with the cation resin, it is very difficult to fully regenerate a cation resin that has been exhausted by contact with sodium hydroxide. Therefore, the exhausted cation resin can contribute to sodium leakage.
Third, because conventional backwashing is employed to separate the bulk of the anion and cation resins, the size of the cation resin capable of being employed in the mixed resin bed continues to be unduly restricted.
Fourth, the bulk volume of cation resin employed in the mixed bed also continues to be unduly restricted because of the continued use of the fixed anion resin takeoff point in the backwash separation procedure.
Thus, there is a need for a process for separating the bulk of the anion and cation resins that (a) yields better separation of these resins; (b) does not limit either the size or bulk volume of resins that can be employed in a mixed bed; and (c) does not adversely exhaust either the cation or the anion resin.